As an example of this type of suspension devices for vehicles, PTL 1 describes a suspension device that includes shock absorbers that are provided between a left wheel and the vehicle body and between a right wheel and the vehicle body, respectively. In addition to the shock absorbers, the suspension device includes a damping mechanism including: a left hydraulic cylinder that is provided between the left wheel and the vehicle body; a right hydraulic cylinder that is provided between the right wheel and the vehicle body; a first fluid path that connects an upper cylinder chamber of the left hydraulic cylinder and a lower cylinder chamber of the right hydraulic cylinder together in communication with each other; a second fluid path that connects an upper cylinder chamber of the right hydraulic cylinder and a lower cylinder chamber of the left hydraulic cylinder together in communication with each other; a third fluid path that connects the first fluid path and a reservoir tank together in communication with each other; a fourth fluid path that connects the second fluid path and the reservoir tank together in communication with each other; and variable throttles that are provided in the third and fourth fluid paths, respectively. The suspension device also includes a control mechanism that adjusts the damping coefficient by controlling the positions (opening degrees) of the variable throttles, depending on how much the wheels and the vehicle body are vertically moved relative to each other.
In this suspension device for a vehicle, the shock absorber and the hydraulic cylinder type damping mechanism are arranged side by side, and therefore, the structure around the wheel is disadvantageously complicated. Moreover, it is necessary to detect the relative vertical movement (an amount, speed, etc.) of the wheel and the vehicle body, and based on the result of the detection, control the damping mechanism. Therefore, it is likely to disadvantageously take a lot of time and effort to control the device.
Furthermore, PTL 2 describes a suspension device for a four-wheel drive vehicle that includes a right wheel-side hydraulic cylinder and a left wheel-side hydraulic cylinder, the inside of each cylinder being partitioned into an upper oil chamber and a lower oil chamber by a piston. Each cylinder is interposed between a wheel and the vehicle body with the cylinder main body being coupled to one of the wheel and the vehicle body and the piston being coupled to the other one of the wheel and the vehicle body. The upper oil chamber of the right wheel-side hydraulic cylinder and the lower oil chamber of the left wheel-side hydraulic cylinder are made to communicate with each other via a first communication path. The lower oil chamber of the right wheel-side hydraulic cylinder and the upper oil chamber of the left wheel-side hydraulic cylinder are made to communicate with each other via a second communication path. A throttle member for generating a damping force is disposed in each of the first and second communication paths. Also, a damper having a cylinder whose inside is partitioned into a high pressure gas chamber and an oil chamber by a free piston, the oil chamber being made to communicate with a connection port via a throttle, is connected to each of the first and second communication paths.
This suspension device is configured to achieve favorable damping characteristics by damping expansion and contraction of the hydraulic cylinders due to rolling, bouncing, or the like of the vehicle body with resistance that a hydraulic fluid undergoes when passing through the throttle members in the first and second communication paths and the throttles of the dampers. However, in this suspension device, if internal leakage of the hydraulic fluid in the upper or lower chamber of the left wheel-side or right wheel-side hydraulic cylinder occurs as a result of repeated operating cycles of these cylinders, the hydraulic balance between the left and right hydraulic cylinders is lost, and thus the vehicle may disadvantageously tilt to the left or right.